This section provides background information related to the present disclosure which is not necessarily prior art.
An automatic transmission provides smooth shifting between gears. Planetary and dual-clutch transmissions are two alternative forms of automatic transmissions. A planetary automatic transmission may have lower gear sliding-related losses due to smaller gear modules (having shorter gear teeth) than those used on a dual-clutch transmission. In addition, a planetary automatic transmission can often utilize lower viscosity lubricant for gear and clutch protection as compared to a dual-clutch transmission. Nonetheless, a planetary automatic transmission is usually less efficient than a dual-clutch transmission due to the fact that a planetary automatic transmission utilizes a larger number of wet clutches (i.e., friction-plate clutches) than does a dual-clutch transmission, which typically incorporates switch-type clutches such as dog clutches.
Clutch inefficiency in a planetary automatic transmission comes primarily from drag-related losses of spinning, disengaged friction plates of the wet clutches. While drag-related losses can be minimized by providing adequate spacing between the friction plates, such additional spacing typically results in longer shift times.
A dog clutch or other switch-type clutch arrangement may provide a more efficient clutch configuration in its disengaged state. However, use of such switch-type clutches does not allow for appropriate gradual energy dissipation, thereby resulting in abrupt gear shifts.
In sum, wet clutches provide a longer shift time when compared to switch-type clutch arrangements, and, thus, a smoother shift, but suffer from the disadvantage of spinning losses when disengaged.